Magnetic recording (“MR”) media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval purposes, typically in disk form. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of the grains of magnetic material.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, of the magnetic media However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits. In this regard, the perpendicular recording media have been found superior to the more common longitudinal media in achieving very high bit densities.
As indicated above, much effort has been directed toward enhancing the density of data storage by both types of magnetic media, as well as toward increasing the stability of the stored data and the ease with which the stored data can be read. For example, it is desirable to develop magnetic media having large magnetic coercivities, Hc, since the magnetic moments of such materials require large magnetic fields for reorientation, i.e., switching between digital 1s and 0s. Thus, when the magnetic medium has a large coercivity Hc, exposure of the magnetic medium to stray magnetic fields, such as are generated during writing operations, is less likely to corrupt data stored at adjacent locations.
The density with which data can be stored within a magnetic thin-film medium for perpendicular recording is related to the perpendicular anisotropy (“Ku” or “K⊥”) of the material, which reflects the tendency for the magnetic moments to align in the out-of-plane direction. Thin-film magnetic media having high perpendicular anisotropies have their magnetic moments aligned preferentially perpendicular to the plane of the thin film. This reduces the transition length between the areas with magnetic moments of opposite direction, thereby allowing a larger number of magnetic bits (domains) to be packed into a unit area of the film and increasing the areal density with which data can be stored.
A large perpendicular anisotropy is also reflected in a larger value of the magnetic coercivity Hc, since the preferential out-of-plane alignment of the magnetic moments raises the energy barrier for the nucleation of a reverse magnetization domain, and similarly, makes it harder to reverse the orientation of the magnetic domains by 180° rotation. Further, the magnetic remanence of a medium, which measures the tendency of the magnetic moments of the medium to remain aligned once the magnetic field is shut off following saturation, also increases with increasing K⊥.
A promising new class of materials for use as the active recording layer of perpendicular MR media includes multilayer magnetic “superlattice” structures comprised of a stacked plurality of very thin magnetic/non-magnetic layer pairs, for example cobalt/platinum (Co/Pt)n and cobalt/palladium (Co/Pd)n multilayer stacks. As schematically illustrated in the cross-sectional view of FIG. 1, such multilayer stacks or superlattice structures 10 comprise n pairs of alternating discrete layers of Co (designated by the letter A in the drawing) and Pt or Pd (designated by the letter B in the drawing), where n is an integer between about 10 and about 30. Superlattice 10 is typically formed by a suitable thin film deposition technique, e.g., sputtering, and can exhibit perpendicular magnetic isotropy arising from metastable chemical modulation in the direction normal to the substrate (not shown in the figure for illustrative simplicity). Compared to conventional cobalt-chromium (Co—Cr) magnetic alloys utilized in magnetic data storage/retrieval disk applications, such (Co/Pt)n and (Co/Pd)n, multilayer magnetic superlattice structures offer a number of performance advantages. For example, a (Co/Pt)n multilayer stack or superlattice 10 suitable for use as a magnetic recording layer of a perpendicular MR medium can comprise n Co/Pt or Co/Pd layer pairs, where n=about 10 to about 30, e.g., 13, and wherein each Co/Pt layer pair consists of an about 3 Å thick Co layer adjacent to an about 8 Å thick Pt or Pd layer; for a total of 26 separate (or discrete) layers. Such multilayer magnetic superlattice structures are characterized by having a large perpendicular anisotropy, high coercivity Hc, and a high squareness ratio of a magnetic hysteresis (M—H) loop measured in the perpendicular direction. By way of illustration, (Co/Pt)n and (Co/Pd)n multilayer magnetic superlattices, wherein n=about 10 to about 30 Co/Pt or Co/Pd layer pairs having thicknesses as indicated supra and fabricated, e.g., by means of techniques disclosed in U.S. Pat. No. 5,750,270, the entire disclosure of which is incorporated herein by reference, exhibit perpendicular anisotropies exceeding about 2×106 erg/cm3; coercivities as high as about 5,000 Oe; squareness ratios (S) of a M—H loop, measured in the perpendicular direction, of from about 0.85 to about 1.0, and carrier-to-noise ratios (“CNR”) of from about 30 dB to about 60 dB.
A key advance in magnetic recording (MR) technology which has brought about very significant increases in the data storage densities of magnetic disks has been the development of extremely sensitive magnetic read/write devices which utilize separate magnetoresistive read heads and inductive write heads. The magnetoresistive effect, wherein a change in electrical resistance is exhibited in the presence of a magnetic field, has long been known; however, utilization of the effect in practical MR devices was limited by the very small magnetoresistive response of the available materials. The development in recent years of materials and techniques (e.g., sputtering) for producing materials which exhibit much larger magnetoresistive responses, such as Fe—Cr multilayer thin films, has resulted in the formation of practical read heads based upon what is termed the giant magnetoresistive effect, or “GMR”. Further developments in GMR-based technology have resulted in the formation of GMR-based head structures, known as GMR “spin valve” heads, which advantageously do not require a strong external magnetic to produce large resistance changes, and can detect weak signals from tiny magnetic bits.
The use of such GMR-based spin valve heads can significantly increase the areal density of MR media and systems by increasing the track density, as expressed by the number of tracks per inch (“TPI”) and the linear density, as expressed by the number of bits per inch (“BPI”), where areal density=TPI×BPI. Currently, GMR-based spin valve heads are utilized for obtaining areal recording densities of more than about 10 Gb/in2; however, even greater recording densities are desired. A difficulty encountered with further increase in the BPI of conventional MR media is that the smaller grain sizes necessary for increase in the BPI results in thermal instability of the media due to exceeding the superparamagnetic limit.
As indicated supra, multilayer magnetic superlattice structures can provide several advantages vis-à-vis conventional thin-film MR media, when utilized in fabricating very high areal density MR media. Specifically, multilayer magnetic superlattice MR media exhibit higher interfacial (i.e., perpendicular) anisotropy (Ku or K⊥) than conventional thin film MR media, such that the value of KuV/kT is greater (where Ku=anisotropy constant; V=volume in cm3; k=Boltzmann's constant; and T=absolute temperature, ° K); increased thermal stability; and very high values of perpendicular coercivity Hc, i.e., >103 Oe. In this regard, the very high values of Hc attainable with multilayer magnetic superlattice structures translates into a significant increase in BPI, and thus a substantial increase in areal recording density.
Multilayer magnetic superlattice structures utilized in MR media can have either an ordered structure forming a true superlattice, or a disordered structure, variously termed a “non-superlattice multilayer” or a “pseudo-superlattice structure”. In (Co/Pt)n and (Co/Pd)n multilayer superlattice structures having utility in high areal density MR media, the interfaces between the Co and Pt (or Pd) layers incur surface interactions, such as for example, spin-orbit coupling. Because Co and Pt (or Pd) have different electronic shell structures or configurations, spin-orbit coupling occurs between the spin and orbital motions of the electrons. When an external magnetic field is applied for reorienting the spin of an orbiting electron, the orbit is also reoriented. However, because the orbit is strongly coupled to the metal lattice, the spin axis of the electron resists rotation. The energy required for reorienting (i.e., rotating) the spin system of a magnetic domain away from the easy direction is termed the anisotropy energy. The stronger the spin-orbit coupling,or interaction, the higher the anisotropy energy and the coercivity Hc.
It is believed that an increase in the interfacial anisotropy of (Co/Pt)n, and (CO/Pd)n, multilayer magnetic superlattice structures, as by an increase in the disorder or “broken symmetry” at each of the Co/Pt or Co/Pd interfaces, can result in the obtainment of very high perpendicular magnetic coercivities Hc necessary for fabricating ultra-high areal density, thermally stable MR media, i.e., perpendicular coercivities on the order of about 104 Oe. It is further believed that the amount or degree of disorder or “broken symmetry” at each of the Co/Pt or Co/Pd interfaces of sputtered (Co/Pt)n and (Co/Pd)n superlattices can be increased by substituting the conventionally employed Ar sputtering gas with a higher atomic weight sputtering gas, e.g., Kr or Xe, thereby providing bombardment of the deposited films with ionized particles having greater momentum, resulting in a greater amount of lattice disruption, disorder, and/or “broken symmetry” at the layer interfaces. One such sputtering process utilizing Kr and/or Xe for forming a (Pt/Co)n multilayer film, for use in magneto-optical (“MO”) recording medium, is disclosed in U.S. Pat. No. 5,106,703 to Carcia, the entire disclosure of which is incorporated herein by reference. However, the sputtering process disclosed therein utilized a relatively low sputtering power density and a relatively large target-to-substrate spacing, and the highest value of the perpendicular magnetic coercivity of the films obtained in any of the illustrative Examples was less than about 1,500 Oe, with the majority of (Co/Pt)n multilayer magnetic films exhibiting coercivities below about 103 Oe. Such values of Hc are considered too low for obtaining MR media with ultra-high areal recording/storage densities on the order of Gbit/in2.
Accordingly, there exists a need for improved methodology for forming, by sputtering, (Co/Pt)n and (Co/Pd)n multi layer magnetic superlattice structures exhibiting very high perpendicular magnetic coercivities Hc, i.e., as high as about 10,000 Oe, which improved methodology can be easily and readily implemented in a cost-effective manner for fabrication of ultra-high areal recording density MR media. Further, there exists a need for improved perpendicular MR media having ultra-high areal recording densities of from about 100 Gbit/in2 to about 600 Gbit/in2 and improved thermal stability, which media can be fabricated in an economical fashion utilizing conventional manufacturing equipment.
The present invention, wherein (Co/Pt)n and (Co/Pd)n multilayer magnetic superlattice structures having very large values of perpendicular coercivities Hc as high as about 10,000 Oe, are formed by sputtering of Co and Pt or Pd targets in an atmosphere containing at least one of Kr and Xe sputtering gases at relatively high power densities and small target-to-substrate spacings, effectively addresses and solves problems attendant upon the use of conventional sputtering techniques for obtaining high quality multilayer magnetic superlattice structures suitable for use in manufacturing improved perpendicular MR media having ultra-high areal recording densities with good thermal stability, while maintaining full compatibility with all aspects of automated MR media manufacturing technology. Further, the methodology provided by the present invention enjoys diverse utility in the manufacture of all manner of films, devices, and products requiring multilayer magnetic thin film coatings and structures exhibiting very high values of perpendicular anisotropy and magnetic coercivity.